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Re: [boost] [Boost-users] Review: Switch library
by Hartmut Kaiser 13 Jan '08

13 Jan '08

Joel de Guzman wrote: > Richard wrote: > > [Please do not mail me a copy of your followup] > > > > boost(a)lists.boost.org spake the secret code > > <fmblls$ees$1(a)ger.gmane.org> thusly: > > > >> I also urge Steven to pursue the dynamic_switch idea I > posted which > >> employs perfect hashing at compile time whereby allowing > equivalent > >> or faster than a switch dispatch at runtime where the cases are > >> dynamic (instead of compile time constants). > > > > I just want to say that I think this approach is so > amazingly cool, my > > socks are not only blown off, but orbiting the planet :-). > > Yes, it's so cool. I believe it can be done. A well designed > compiler implements switch using perfect (or near perfect) > hashing anyway. Seems though that some compilers still don't. > > Here's the relevant link (with attached experiment code): > > http://lists.boost.org/Archives/boost/2004/08/69787.php FWIW, I did a lot of work related to perfect hash generation lately (up 100 million entries in the hash table) and I second Joels request. Perfect hashs are a _very_ fast alternative to sequenced if/else if chains. The beauty of this is, that it is possible to generate perfect hashs for any type of key, not only for integer values, which again is of particular interest in dynamic switch scenarios. And I'ld be happy to help with this. Regards Hartmut

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Re: [boost] [Boost String Algorithms library] ireplace_all bug with character arrays
by Thorsten Ottosen 13 Jan '08

13 Jan '08

shunsuke skrev: > Richard Turrin wrote: >> Hi, >> >> I've encountered a bug with ireplace_all from the Boost String Algorithms library for version 1.35. If I pass a character array for the Format argument, the function ignores the null terminator and instead uses the entire array minus the last character to replace the occurrences of the Search string in the Input parameter. However, if I provide a pointer to a character array, the string replacement is done as expected. >> >> When I stepped into the code, I came across the following lines (excerpt from /trunk/boost/range/as_literal.hpp): >> >> template< class Char, std::size_t sz > >> inline iterator_range<const Char*> as_literal( const Char (&arr)[sz] ) >> { >> return boost::make_iterator_range( arr, arr + sz - 1 ); >> } >> >> There is no check for a null terminator. Nor did I find one after this function returned. I think the author here assumed that a caller would always pass in a character array of the right size. > > Character array seems to have three views. > > 1. array: SIZE("ab\0c") == 5; > 2. literal: SIZE("ab\0c") == 4; > 3. null-terminated: SIZE("ab\0c") == 2; > > Both 2. and 3. is forced into boost::as_array. > "One name, two facilities" usually makes trouble. > IMO, `as_c_str` should have been added for 3. I agree we could add a new function. Can we come up with a better name than as_c_str()? Or should we simply make as_literal() scan the array? -Thorsten

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Re: [boost] Review: Switch library
by Joel de Guzman 13 Jan '08

13 Jan '08

Tobias Schwinger wrote: > Joel de Guzman wrote: >> Richard wrote: >>> [Please do not mail me a copy of your followup] >>> >>> boost(a)lists.boost.org spake the secret code >>> <fmblls$ees$1(a)ger.gmane.org> thusly: >>> >>>> I also urge Steven to pursue the dynamic_switch idea I posted >>>> which employs perfect hashing at compile time whereby allowing >>>> equivalent or faster than a switch dispatch at runtime where >>>> the cases are dynamic (instead of compile time constants). >>> I just want to say that I think this approach is so amazingly cool, my >>> socks are not only blown off, but orbiting the planet :-). >> Yes, it's so cool. I believe it can be done. A well designed >> compiler implements switch using perfect (or near perfect) >> hashing anyway. Seems though that some compilers still >> don't. > > Another interesting approach could be "adaptive switch" as explained here: > > http://osdir.com/ml/parsers.spirit.devel/2006-05/msg00025.html > > (a self-optimizing alternative parser). Maybe it can be combined with > hashing techniques. Yeah, it's a very interesting idea! Regards, -- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net

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Re: [boost] Review: Switch library
by Joel de Guzman 13 Jan '08

13 Jan '08

Joel de Guzman wrote: >> And finally, every review should answer this question: >> >> * Do you think the library should be accepted as a Boost library? >> Be sure to say this explicitly so that your other comments don't >> obscure your overall opinion. > > Not at the moment. I think we need a more thorough discussion on > alternative interfaces. We also need to discuss the issues > that were raised in the review. I'm eager to hear Steven's > replies. He seem to be a bit too quiet? > > I'm really tempted to say "yes" and let Steven address the concerns > raised (including mine). I'm very confident in Steven's abilities. > He's one of those who still gives me the "oooh" feeling with his > code. And, I really *NEED* such a switch utility now and not later. > > So, please take this as a soft "no" vote for now. I encourage > Steven to get more involved in the discussion and consider > all the issues raised. As soon as these matters are ironed out, > fire up another review ASAP. Oh, perhaps I'd like to ask a review extension. If Steven can reply to my concerns and is willing to address them, I might still change my vote to a yes. I really need this facility now! Bottom line: more discussion please! This is a small library, yet it is (to me) very important. Let's get the design correct. Regards, -- Joel de Guzman http://www.boost-consulting.com http://spirit.sf.net

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Re: [boost] [Range] Misc comments
by bwood 13 Jan '08

13 Jan '08

Thorsten Ottosen wrote: >brass goowy skrev: >> utility_class.html says, "Recall that many default >> constructed iterators are singular..." >> >> I didn't find other uses of the word singular in the >> documentation and am not sure what you mean. Are you >> saying they are not meant to be range endpoints? > > From the standard 24.1: > >"Iterators can also have singular values that are >not associated with any container. [ Example: After the declaration of >an uninitialized pointer x (as with int* x;), x >must always be assumed to have a singular value of a pointer. ?end >example ] Results of most expressions are undefined >for singular values; the only exceptions are destroying an iterator that >holds a singular value and the assignment of a >non-singular value to an iterator that holds a singular value. In this >case the singular value is overwritten the same way >as any other value. Dereferenceable values are always non-singular." > OK, thanks. >> I think the "sub" in the class name sub_range is kind of >> misleading. The documentation has the following: >> "Imagine that we have an algorithm that searches for a >> sub-string in a string. The result is an iterator_range, >> that delimits the match. We need to store the result from >> this algorithm. Here is an example of how we can do it with >> and without sub_range >> >> std::string str("hello"); >> iterator_range<std::string::iterator> ir = find_first( str, "ll" ); >> sub_range<std::string> sub = find_first( str, "ll" );" >> >> The docs also say, "The iterator_range class is templated on an >> Forward Traversal Iterator and should be used whenever fairly >> general code is needed. The sub_range class is templated on an >> Forward Range and it is less general, but a bit easier to use >> since its template argument is easier to specify. The biggest >> difference is, however, that a sub_range can propagate >> constness because it knows what a corresponding const_iterator is." >> >> The sample code that has a sub-string with either an >> iterator_range or sub_range and the description of the >> two classes don't give much basis for sub being in the >> name. > >So you would just call it range? Possible, but difficult to do now. > I prefer range to sub_range. It isn't that important to me, but I suggest you make some comment in the documentation about the matter. That might lead to some further ideas. >> I didn't find anything about serialization in the docs. >> iterator_range has an operator<<, but understandably no >> operator>>. Do you plan to add something in this area? > >No. Don't Boost.Serialization have support for serializing a [begin,end) >range? Not to my knowledge. Even if B.Ser had that, I think using it would require what you are encouraging people to get away from with the Range library: passing around iterators. Brian Wood Ebenezer Enterprises www.webebenezer.net

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Switch library review - summary of discussion so far
by Stjepan Rajko 12 Jan '08

12 Jan '08

I will try to summarize the discussions happening so far on the Switch library under review - maybe this will be helpful to those considering joining the discussion and submitting a review. * Documentation All reviewers have noted that the documentation could be improved - e.g., additional examples, performance stats, as well as more explanation of the types involved and how they are used. * Return type Tobias Schwinger suggested that the return type should be specified explicitly in the switch_ call, rather than inferring it from the function object, and this is now a part of the recent designs being discussed. Joel de Guzman also suggested that if a version of switch_ should infer the return type, that return type should be a Boost.Variant of the types returned by each of the cases. * Default default behavior If the default behavior is left unspecified, switch_ should invoke the default constructor of the return type in the default case, rather than throwing an exception. This was suggested by Tobias Schwinger after the issue was raised by Dan Marsden. Alexander Nasonov has also indicated that a throwing default case can cause inefficiency, even when it is guaranteed not to be invoked. * Fall-through Alexander Nasonov pointed out the lack of fall-through capability in the submitted implementation. Fall-through has been incorporated into the recent designs being discussed. * Allowing void-returning function objects for the default case with a non-void returning switch Tobias Schwinger mentioned the possibility of allowing default case function objects of void return type even when the switch return type is non-void, as long as the function object call does not return. This would be useful, for example, if the function object is guaranteed to throw. There is still some discussion on whether determining if the return type is void can be done efficiently. * Design There has been a lengthy discussion on the underlying design of the library, what it should offer, and whether the name of the library is appropriate. There are two types of designs that have been put forward as being appropriate and/or necessary for a Boost.Switch library. On one hand, there is the submitted design (labeled "A" in recent postings), which takes the switch cases in an MPL sequence and a single function object containing implementations of all cases. Joel de Guzman has proposed another design (labeled "B"), in which the switch cases are specified separately and allow a different function to be used for each case. Design "A" is better suited for situations in which the number of cases is determined by a compile time constant, while "B" is more appropriate for many use cases, and provides more flexibility and a more familiar syntax. It seems that a consensus has been reached that this library could offer either both interfaces in different layers, or an interface that can be used in different ways for different uses (based on a common concept capturing case sequences). (Apologies if I have missed or misinterpreted something) More reviews/votes would be extremely welcome! Regards, Stjepan

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[Range] Misc comments
by brass goowy 12 Jan '08

12 Jan '08

The Intro (intro.html) says, "Below are given a small example..." I think "Here is a small example..." would work. On that same page and other pages this is found: "Warning: support for null-terminated strings is deprecated and will disappear in the next Boost release (1.34)." utility_class.html says, "Recall that many default constructed iterators are singular..." I didn't find other uses of the word singular in the documentation and am not sure what you mean. Are you saying they are not meant to be range endpoints? In the section on portability it says: "Notice also that some compilers cannot do function template ordering properly. In that case one must rely of range_result_iterator and a single function definition instead of overloaded versions for const and non-const arguments." Probably that should be "rely on range_result_iterator..." The history and acknowledgement page says, "The library have been under way for a long time..." Should be, "The library has been under..." In iterator_range.hpp a range is often declared with the name 'r' and then there is code like r.begin and r.end. In my opinion those are distractingly similar to rbegin and rend. I think something like rg.begin and rg.end would be an improvement. I think the "sub" in the class name sub_range is kind of misleading. The documentation has the following: "Imagine that we have an algorithm that searches for a sub-string in a string. The result is an iterator_range, that delimits the match. We need to store the result from this algorithm. Here is an example of how we can do it with and without sub_range std::string str("hello"); iterator_range<std::string::iterator> ir = find_first( str, "ll" ); sub_range<std::string> sub = find_first( str, "ll" );" The docs also say, "The iterator_range class is templated on an Forward Traversal Iterator and should be used whenever fairly general code is needed. The sub_range class is templated on an Forward Range and it is less general, but a bit easier to use since its template argument is easier to specify. The biggest difference is, however, that a sub_range can propagate constness because it knows what a corresponding const_iterator is." The sample code that has a sub-string with either an iterator_range or sub_range and the description of the two classes don't give much basis for sub being in the name. In that last paragraph I quoted, there are two places where it says, "an Forward." I think those should be "a Forward." And swapping "is" and "however" in the last sentence would make it easier to read. I didn't find anything about serialization in the docs. iterator_range has an operator<<, but understandably no operator>>. Do you plan to add something in this area? iterator_range<list<int>::iterator> it_rng; oarch << it_rng; // would need to send the size .................... vector<int> vec; iarch >> vec; That seems useful. Brian Wood Ebenezer Enterprises www.webebenezer.net "They that trust in the L-rd shall be as mount Zion, which cannot be removed, but abides for ever." Psalm 125:1

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XML
by Adam Badura 11 Jan '08

11 Jan '08

It seems to me that Boost lacks "typical XML parser" (by that I mean something offering DOM and SAX parsing as well as validation and maybe some other features). I did not look at serialization that much but I suspect that it does not offer such properties. Why is that? Any technical reasons? Or maybe "political" reasons? Or perhabs simply no one did it? Adam Badura

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Trac open bugs - by assignee
by Marshall Clow 11 Jan '08

11 Jan '08

[ 195 open bugs today, 194 yesterday ] If you are maintaining a library, and have not logged into the trac system at <http://svn.boost.org/trac/boost/>, please do so ASAP. Until you do that, no tickets can be assigned to you, and will remain under "None". Instructions on working with tickets can be found at: <http://svn.boost.org/trac/boost/wiki/TicketWorkflow> and a mapping of user names to "real names" can be found at: <http://svn.boost.org/trac/boost/report/16> Bug summary 18 az_sw_dude 17 dgregor 15 djowel 12 doug_gregor 12 agurtovoy 11 grafik 11 dave 11 None 8 vladimir_prus 8 jsiek 7 johnmaddock 7 anthonyw 6 rogeeff 5 witt 4 bemandawes 3 pdimov 3 nesotto 3 jmaurer 3 igaztanaga 3 ebf 3 danielw 2 urzuga 2 turkanis 2 pavol_droba 2 nasonov 2 matias 2 danieljames 2 <Blank> 1 t_schwinger 1 speedsnail 1 shammah 1 samuel_krempp 1 marshall 1 kaalus 1 hljin 1 hkaiser 1 dlwalker 1 burbelgruff 1 aaron_windsor <Blank> 1307 [lambda] invalid boostbook <Blank> 1434 Boost.Preprocessor_ small fix for the documentation None 1068 Mersenne twister disables streaming for Visual C++ None 1094 Finding the correct library to link (feature request for pkg-config support)h None 1222 lambda::ret<T> should declare result_type. None 1247 Typos in "boost/random/additive_combine.hpp"_ lines 68-69 None 1270 uniform_smallint doesn't compile with lagged_fibonacci random number generators None 1407 Boost is doesn't install on MacPorts using Leopard None 1493 SVN files making it through to install None 1515 [lambda] C4512 warnings from VC8.0 None 1544 uniform_01 w/ref template param causes ref to ref error None 1546 Method boost::random::uniform_int::operator()(engine_ value) is missing None 1548 [variant][docs]typo: formal arg_ operand_ should be v in times_two arglist aaron_windsor 778 top-level configure is broken agurtovoy 1001 gcc 4.1.2 segfaults on mpl/test/apply.cpp agurtovoy 1051 MPL header ordering triggers bug agurtovoy 1099 unable to iterate through a "non-explicitly-declared" mpl::set agurtovoy 1215 Boost 1.34.1_ mpl: Wrong workaround detection using MinGW32 (or.hpp_ line #33) agurtovoy 1317 has_xxx randomly broken in msvc agurtovoy 1469 [mpl] nested typedef of set has two different meanings agurtovoy 1476 always<> is not nullary. agurtovoy 1528 g++ 4.3.0 reports error because of redeclaration of next agurtovoy 1549 Boost.MPL: 'index_of' algorithm is not documented agurtovoy 1568 duplicate include guards agurtovoy 588 mpl::remove compile error with gcc 4.1.0 agurtovoy 640 documentation mistake anthonyw 1154 tss destruction segfaults in conjunction with dlopen/dlclose anthonyw 1168 [thread] tss crashes randomly anthonyw 1268 xtime_cmp changes to deal with overflow were reverted anthonyw 1470 Boost.Thread: Compilation errors with non-__cdecl calling convention anthonyw 1547 [thread] error C2065: 'NULL' : undeclared identifier in thread_primitives.hpp anthonyw 876 xtime documentation incomplete anthonyw 881 errors when build with Apache stdcxx az_sw_dude 1028 typo in boost header: date_time/time_defs.hpp az_sw_dude 1078 from_iso_string cannot read the output from to_iso_string az_sw_dude 1098 Urgent libboost-date-time1.33.1 : bugs on ia64 ? az_sw_dude 1147 date_time library contains global #define ERROR az_sw_dude 1160 warning: month_format shadows a member of this az_sw_dude 1162 warning: ticks shadows a member of this az_sw_dude 1289 posix_time::time_facet forgets to pass a_ref argument to base class az_sw_dude 1299 change in typedef stringstream_type in format_date_parser.hpp causing compilation error az_sw_dude 1306 invalid docbook in date_period.xml az_sw_dude 1384 Linking problem with date_time library on Visual C++ 2008 Express az_sw_dude 1386 date_time documentation Jamfile breaks building of boost docs az_sw_dude 1524 format_date_parser.hpp(221) : cannot convert parameter 1 az_sw_dude 287 wrong usage of ios_base::narrow az_sw_dude 316 date_time type conversion warning az_sw_dude 604 from_ftime incorrectly processes FILETIME filled with zeros az_sw_dude 642 operator<< for gregorian::date_duration not found az_sw_dude 889 Insane Dependencies az_sw_dude 890 date_time library supports only the current dst rule bemandawes 1378 filesystem path invalid name for hidden files (LINUX) bemandawes 1389 filesystem::ostream << boost::format generates operator ambigious error (in MSVC++) bemandawes 1458 Darwin Tiger throws std::runtime_error: locale::facet::_S_create_c_locale name not valid bemandawes 752 directory_iterator doesn't work with catch burbelgruff 1503 typeof bug (big integral constant) danieljames 1554 When BOOST_HASH_NO_EXTENSIONS is defined hash doesn't support new string types. danieljames 1555 When BOOST_HASH_NO_EXTENSIONS is defined hash doesn't support boost::system::error_code danielw 1044 Argument pack inaccessable in a functions return type danielw 1054 Parameter takes an "infinite" amount of time to compile with the Intel compiler. danielw 1097 keyword.hpp generates many warnings C4180: qualifier applied to function type has no meaning; ignored dave 1010 linking boost.python on freebsd dave 1019 iterator_facade::pointer should be the return type of operator-> (DR 445) dave 1085 Python rules for compiling in Mac OS X dave 1179 [boost.python] can not export a union in VC2005sp1 dave 1181 [boost.python] can modify enum value dave 1387 Review / update docs to reflect changes dave 1460 Python classes with multiple bases are convertible only to the first class in the lists dave 1482 boost::python::throw_error_already_set should be marked noreturn dave 1516 [python] Failing VC9 regression tests dave 865 Use of C++0x keyword as identifier dave 910 gcc strict-aliasing problems with python dgregor 1003 [iostreams] copy-paste error in typedefs for wchar multichar_filters dgregor 1139 [iostreams] Regex filter segfaults when zero matches found dgregor 1140 boost::iostreams linked_streambuf bug dgregor 1149 Minor doc error in basic_null_device dgregor 1164 Floyd Warshall broken with unsigned edge weights dgregor 1185 iostreams::direct_streambuf<T_Tr>::is_open always returns false if output iterator is not null dgregor 1192 [boostbook] problem when two files have the same name dgregor 1199 configure doesn't work with path containing whitespaces (MacOS X) dgregor 1232 mapped_file_source::is_open() is broken under windows dgregor 1310 [result_of] const-qualified function pointer doesn't work dgregor 1326 Unable to check graph isomorphism using LEDA adapter dgregor 1391 dominator_tree_test on Borland/CodeGear outputs overly long line dgregor 1394 Translate libs/graph/build/python/Jamfile to BBv2 dgregor 1395 Write Tests for Python Bindings dgregor 556 Bundled graph properties dgregor 732 Johnson All-Pairs needs better "no path" information dgregor 736 Const correctness violation djowel 1170 Emit Warnings instead of # symbols in the syntax highlighter djowel 1171 Quickbook title output djowel 1172 non-utf djowel 1176 Space after inline code djowel 1206 Escaping from within code? djowel 1273 CR+LF newlines in position_iterator djowel 1276 spirit problem with _GLIBCXX_DEBUG djowel 1277 Closing section like [pre with } causes Quickbok to crash djowel 1280 sections in imported source files get put inside <para> elements in docbook output djowel 1281 Quickbook produces invalid BoostBook/DocBook XML djowel 1324 tuple should have a swap() that operates element-by-element djowel 1396 wrong result_of invocation around transform_view djowel 1436 [quickbook] inline code inside headings djowel 1484 Quickbook produces illegal docbook for some docinfo fields. djowel 1500 [Spirit/Phoenix] C4512 Compiler warnings from VC8 dlwalker 613 boost/crc.hpp uses non-standard conforming syntax doug_gregor 1021 [Graph][reverse_graph] Problem using reverse_graph adapter with an adapted user defined graph doug_gregor 1076 compile error: disconnecting struct with operator()() doesn't work in VS2005 SP1 doug_gregor 375 LEDA graph adaptors do not handle hidden nodes properly doug_gregor 402 random_vertex/random_edge are unnecessarily inefficient doug_gregor 403 Document copy_component doug_gregor 447 reverse_graph and constness of property maps doug_gregor 708 Dijkstra no_init version should not require VertexListGraph doug_gregor 733 Numbered headers don't work with "preferred" syntax doug_gregor 735 Fruchterman-Reingold grid performance can be improved doug_gregor 738 Memory leaks with signal::connect? doug_gregor 829 Implicit graph not implement doug_gregor 852 Problem with Boost and GCC 4.1 ebf 1507 [variant] C4512 warnings from VC8.0 ebf 965 [doc] boost::variant tutorial - final example uses v1_v2 should be seq1_seq2 ebf 993 Variant should use least common multiple of alignments_ rather than maximum grafik 1041 Cygwin requires PATH setting. grafik 1062 Cygwin install stage fails with combination of patches grafik 1322 trac rejects patch attachment as spam grafik 1417 Regresson tests hang in Darwin (Tiger) grafik 1430 Regression fails under Cygwin grafik 1433 Trac's spam detection rejects too many valid messages grafik 1512 Broken link to vacpp in bjam docs. grafik 1513 Broken link to bcpp in bjam docs. grafik 616 Boost Jam_ and non english directorys grafik 959 linking fails when Boost is compiled with STLport 5.1.* on Linux (names mismatch) grafik 986 Problem building Python modules on boost 1.34.0 hkaiser 1079 boost-1.34.0 fails to compile. concept checks not satisfied. hljin 1522 bit_range friend decl in bit_aligned_pixel_reference.hpp needs to be fully qualified for gcc 4.3 mainline igaztanaga 1080 boost::interprocess win32 global file mapping issue igaztanaga 1210 interprocess::barrier hangs during second use under linux (boost 1.33.1) igaztanaga 1390 Interpocess: managed shared memory failing completely under OS X jmaurer 351 Diff in state of mersenne_twister gen between GCC3.41 & CW9 jmaurer 599 uniform_01 copies engine instead of using a reference jmaurer 819 uniform_int<> with type's maximum availaible range error johnmaddock 1081 Empty string rejected as perl-style regex johnmaddock 1314 Using SSE-intrinsics in VC8 causes static assert in type_with_alignment.hpp johnmaddock 1411 Fix BOOST_PARTIAL_SPECIALIZATION_EXPLICT_ARGS johnmaddock 1440 libs/config/test/link/Jamfile.v2 confuses process_jam_log johnmaddock 1446 POSIX API doesn't use C locale johnmaddock 1451 error: use of C99 long long integer constant johnmaddock 1511 Non latin symbols in regexp jsiek 1531 segfault in dynamic_bitset::reference assign operator jsiek 1540 Poisson distribution very slow for large mean (and may also overflow). jsiek 416 Spelling of Edmonds-Karp-Algorithm jsiek 575 Calling subgraph::global_to_local on a root graph jsiek 636 strange compiling problem for transitive_closure jsiek 815 remove_edge core dumps on self-circle jsiek 875 No iterator based constructor for adjacency_matrix jsiek 900 graphml reader namespace handling is broken kaalus 1184 property_tree assignment (operator) bug marshall 1252 [pool] severe overhead with unaligned sizes. matias 1406 Understanding the Tables outdated matias 1539 Missing semi-colon in random library quitck start example nasonov 1220 lexical_cast requires RTTI nasonov 754 boost::any - typeid comparison across shared boundaries nesotto 1284 [range] sub_range assignment issue nesotto 1464 boost_range_array unused in several array_size() functions nesotto 1565 iterator_range disables msvc warning 4996 pavol_droba 1152 rle_example fails pavol_droba 1385 begin must be boost-qualified pdimov 1106 shared_ptr<T> should not convert to shared_ptr<U> when T* doesn't convert to U* pdimov 1444 scoped_ptr::operator== should be equivalent to a.get()==b.get()_ but is currently !!a==!!b pdimov 779 gcc-4.2 atomicity.h location rogeeff 1383 signal.h features not found on Mac OS rogeeff 1489 BOOST_CHECK_CLOSE fails to compile with templated value type rogeeff 1542 [test] compiler errors when building with stdcxx library rogeeff 1560 Performance testing with boost::test rogeeff 1561 Using BOOST_AUTO_TEST_CASE with boost::program_options rogeeff 1566 Missing example files in documentatio samuel_krempp 859 boost::format ignores a user defined locale shammah 386 boost::pool_allocator breaks with vector of vectors speedsnail 1138 Problems with TSS and static thread library t_schwinger 1100 Building boost.function_types fails cvs rev-1.1_ no chdir command in bash turkanis 1523 [iostreams] VC8/VC9 compiler warnings turkanis 1545 [boost/rational.hpp] 'noskipws' is not a member of 'std' urzuga 574 [boost::lambda] Compile error with libstdc++ debug mode urzuga 781 Lambda: (_1 + "y")(string("x")) Doesn't Compile vladimir_prus 1165 more quoting problems vladimir_prus 1274 Useless error message vladimir_prus 1321 invalid docbook in tool documentation vladimir_prus 1527 [program_options] Second (and subsequent) lines in tabbed text not aligned correctly vladimir_prus 1530 Program options documentation - Overview - Syntactic Information example typo vladimir_prus 1537 no value given in 'mysql_password =' vladimir_prus 469 multitoken broken vladimir_prus 689 [program_options] Endless loop with long default arguments witt 1046 Boost.Python quickstart instructions mention nonexistent target witt 583 Fixes for build on IBM pSeries for AIX and Linux witt 834 Homepage regression links witt 957 The "Getting Started" page does not mention the stdlib option witt 973 zip_iterator has value_type == reference -- -- Marshall Marshall Clow Qualcomm, Inc. <mailto:mclow@qualcomm.com> The famous British scientist, Lord Kelvin, said: "When you measure what you are speaking about and express it in numbers, you know something about it, but when you cannot (or do not) measure it, when you cannot (or do not) express it in numbers, then your knowledge is of a meagre and unsatisfactory kind."

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[RFC] Generic non-intrusive Visitor pattern utility
by dizzy 11 Jan '08

11 Jan '08

Hi It is often in my line of work that I need to perform several duties depending on the exact type (dynamic type) of a pointer/reference to base of a class hierarchy. This is done cleanly with a Visitor pattern aproach but usual implementations of Visitor (as far as I could see) are intrusive, meaning they require the original class hierarchy to have some Visitor API (usually in the form of a virtual function taking a pointer/reference to a base Visitor class). I find that undesirable because in my use cases at least, the design of the original class hierarchy does not need to know that it may be visited at some point nor does it care about that. Not to mention it is not very practical to modify the original hierarchy when you find out you need a Visitor (because it may be part of a separate project, you may not even have the sources to it). So I thought I need something non-intrusive to work on any class hierarchy one desires to use Visitor upon. An aproach solving this would be implementing a parallel class hierarchy that is Visitor aware (in the form of a template wrapper class instantiated by with the types visited), thus is nonintrusive but when actually implementing this, I noticed several draw backs: - using virtual functions it still imposes using some base class Visitor object and people have to use inheritance (which I don't think is really needed) - how to wrap the actual type is tricky, you may value aggregate it and then you require a working copy ctor of it (not always available or maybe expensive) or you may store pointers to it and then deal with the overhead of dynamic memory allocation just because of the design and one more indirection (neither of these required in the end) The final (best?) solution I came with now is using RTTI. RTTI's typeid() is by design non-intrusive (actually the way it is implemented by the compiler is intrusive but it happens anyway for any polymorphic type so why not use it) and it does not have any of the problems listed above, the implementation is "small enough" that I can include it here: #include <typeinfo> #include <stdexcept> #include <string> namespace detail { template<typename List> struct VisitorHelper { // empty body to compile error in case of wrong "List" parameter }; template<typename Left, typename Right> struct VisitorHelper< TypeList<Left, Right> > { // notice how we copy "Visit" because it's suposed to be // cheap to copy callback or similar thing template<typename Return, typename Visit, typename Visitee> static Return select(Visit visit, Visitee& visitee) { // typeid will consider the type without qualifiers so we need to // preserve them in conversion // (FIXME: does not work with volatiles) if (typeid(Left) == typeid(visitee)) { return Return(visit(static_cast<typename copy_cv<Visitee, Left>::type&>(visitee))); } else return VisitorHelper<Right>::template select<Return>(visit, visitee); } }; template<> struct VisitorHelper< NullType > { template<typename Return, typename Visit, typename Visitee> static Return select(Visit, Visitee& visitee) { // type not found in the known types list throw std::runtime_error("Type unknown: " + std::string(typeid(visitee).name())); } }; } // List should be a canonical TypeList (without cv qualifiers, // no reference or pointer types, no array types) template<typename List, typename Return, typename Visit, typename Visitee> Return apply_visitor(Visit visit, Visitee& visitee) { return detail::VisitorHelper<List>::template select<Return>(visit, visitee); } Not included here but used in the code are TypeList, NullType and copy_cv. They are easy to guess, TypeList is the usual TypeList (as in mr Alexandrescu's well known book), NullType is a incomplete type just to signal termination of the list and copy_cv is a type function that will add cv qualifiers of a type to another. It is used in the code where if the dynamic type is found, we need to static_cast the base type of the given parameter to that found dynamic type but we should static_cast by keeping the base type cv qualifiers. Use case: struct Base { virtual ~Base() {} }; struct Derived1: Base {}; struct Derived2: Base {}; typedef TypeList<Derived1, TypeList<Derived2, NullType> > VisitedTypes; struct Visitor1 { void operator()(Derived1 const& op) const {} bool operator()(Derived2& op) { return false; } }; struct Visitor2 { bool operator()(Derived1& arg) const { return false; } int operator()(Derived2 const& arg) const { return 10; } }; struct Visitor3 { void operator()(Base const&) {} }; struct Visitor4 { void operator()(Derived1 const&) const {} }; int main() { Visitor1 vis1; Visitor3 vis3; std::auto_ptr<Base> obj(new Derived1); // will call Visitor1::operator()(Derived1&) const; apply_visitor<VisitedTypes, void>(vis1, *obj); // compile time error, cannot convert void to bool result // of Visitor1::operator()(Derived1&) // apply_visitor<VisitedTypes, bool>(vis1, *obj); // will call Visitor2::operator()(Derived1&) const // will convert int return type to bool if "*obj" dynamic type // were Derived2 apply_visitor<VisitedTypes, bool>(Visitor2(), *obj); // will call Visitor3::operator()(Base const&) // even tho that beats the purpose of the Visitor apply_visitor<VisitedTypes, void>(vis3, *obj); // will compile time error, there is no matching function to call // with (Visitor4())(Derived2& argument) // apply_visitor<VisitedTypes, void>(Visitor4(), *obj); } I would like to know if boost implements something similar (I remember there was something related to visitors in the variant library) and if not if you find it interesting and a good addition to boost so I can work on it, performing the required changes to be included with boost. Of course any other technical comments pointing out problems, flaws or trying to improve this code are welcome. Thanks! -- Mihai RUSU Email: dizzy(a)roedu.net "Linux is obsolete" -- AST

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